This invention relates generally to a tool for use in the bonding of wire to semiconductor devices and, more particularly to a bonding tool for bonding of fine wire to bonding pads set at a very fine pitch.
Modern electronic equipment relies heavily on printed circuit boards on which semiconductor chips, or integrated circuits (ICs), are mounted. The mechanical and electrical connections between the chip and the substrate have posed challenges for chip designers. Three well known techniques for interconnecting the IC to the substrate are: wire bonding, tape automated bonding (TAB) and flip-chip.
The most common of these processes is wire bonding. In wire bonding, a plurality of bonding pads are located in a pattern on the top surface of the substrate, with the chip mounted in the center of the pattern of bonding pads, and the top surface of the chip facing away from the top surface of the substrate. Fine wires (which may be aluminum or gold wires) are connected between the contacts on the top surface of the chip and the contacts on the top surface of the substrate. Particularly, the connecting wires are supplied and bonded to the chip and to the substrate through a capillary, a bonding tool further described below.
Capillaries (bonding tools) are used for ball bonding the wire to electronic devices, particularly to bond pads of semiconductor devices. Such capillaries are generally formed from a ceramic material, principally aluminum oxide, tungsten carbide, ruby, zircon toughened alumina (ZTA), alumina toughened zircon (ATZ). Very thin wire, generally on the order of about one mil gold, copper or aluminum wire, is threaded through an axial passage in the capillary with a small ball being formed at the end of the wire, the ball being disposed external of the capillary tip. The initial object is to bond the ball to a pad on the semiconductor device and then to bond a portion farther along the wire to a lead frame or the like. During the bonding cycle, the capillaries perform more than one function.
After the ball is formed, the capillary must first center the ball partly within the capillary for bond pad targeting. With a first bonding step, the ball is bonded to a pad on a semiconductor device. When the capillary touches the ball down on the bond pad, the ball will be squashed and flatten out. As the bond pads are generally made from aluminum, a thin oxide forms on the surface of the bond pad. In order to form a proper bond, it is preferable to break the oxide surface and expose the aluminum surface. An effective way of breaking the oxide is to xe2x80x9cscrubxe2x80x9d the surface of the oxide with the wire ball. The wire ball is placed on the surface of the aluminum oxide and the capillary rapidly moves in a linear direction based on the expansion and contraction of a piezo-electric element placed within the ultrasonic horn to which the capillary is attached. The rapid motion, in addition to heat applied through the bond pad, forms an effective bond by transferring molecules between the wire and the bond pad.
The capillary then handles the wire during looping, smoothly feeding the bond wire both out of the capillary and then back into the capillary. The capillary then forms a xe2x80x9cstitchxe2x80x9d bond and a xe2x80x9ctackxe2x80x9d or xe2x80x9ctailxe2x80x9d bond.
Presently, thermosonic wire bonding is the process of choice for the interconnection of semiconductor devices to their supporting substrates. The thermosonic bonding process is partially dependent upon the transfer of ultrasonic energy from the transducer, attached to a movable bondhead, through a tool, e.g. capillary or wedge, to the ball or wire being welded to the semiconducting device or supporting substrate.
In conventional capillaries (bonding tools), the geometry of the bonding tool and the free air ball (FAB) formed thereby are such that the bonding tool can only be used to bond wires to bonding pads having an interpad spacing (pitch) of greater than 60 microns (0.060 mm; 15.34*10xe2x88x924 in.]. Thus, making them unsuitable for bonding wires to devices produced to meet the higher density requirements of the semiconductor industry. These prior art bonding tools are also unsuitable for handling wire bonds using wire as small a 0.4 mils (10 microns) in diameter. The inventors of the present invention have developed a bonding tool that meets the demands imposed by these high-density devices while maintaining structural integrity of the bonding tool.
FIG. 1A is an illustration of a well-known prior art fine pitch bonding tool 100. Bonding tool 100 has a cylindrical portion 101, and a tapered potion 102 coupled between cylindrical portion 101 and working tip 104. Working tip 104 (at an end of bonding tool 100) has a tip angle of fifteen degrees relative to the longitudinal axis of bonding tool 100. In other words, working tip 104 has an overall angle 106 of 30 degrees. The reduced width of working tip 104 relative to cylindrical portion 101 permits ball bonds to be made on pads having a pitch of about 0.0032 in. without working tip 104 touching an adjacent loop of a bonded wire as explained in U.S. Pat. No. 5,558,270.
FIG. 1B is an illustration of an enlarged sectional view of working tip 104. As shown in FIG. 1B, working face 111 has a face angle 108 of 4 degrees, and tapered portion 104 has an overall angle 118 of 10 degrees. In addition, adjacent working face 111 is first inner chamfer 110, which, in turn, is adjacent second inner chamfer 112. First inner chamfer 110 has chamfer angle 114 of 90 degrees, and connects or continues with second inner chamfer 112 having an angle greater than 60 degrees. These chamfers are designed to guide a fine wire (not shown) into wire hole 116, having a diameter 106, to accommodate wire with a diameter of about 1 mil.
These prior art bonding tool are deficient, however, in that their design is not able to accommodate the ultra fine pitch (30 microns or less) bonding pad requirements placed upon the industry by semiconductor manufacturers. Further, these bonding tools are formed from materials that are unable to withstand the forces and meet the elasticity requirements necessary to provide a bonding tool with working tip dimensions sufficient to meet the needs of the semiconductor industry.
To solve the aforementioned disadvantages of conventional bonding tools, the present invention relates to having a working tip with a diameter less than 39 microns.
The bonding tool comprises a working tip at an end of the bonding tool. The working tip including i) a tapered section having a predetermined angle with respect to the longitudinal axis of the first cylindrical section, ii) a working face with a first annular chamfer formed at an outside portion of an end of the working tip, and iii) a second annular chamfer formed at an inside portion of the end of the working tip, the first and second annular chamfer being adjacent one another; and a substantially cylindrical axial passage coupled to an upper portion of the second annular chamfer.
According to another aspect of the present invention, the second annular chamfer has an overall angle of less than 90xc2x0.
According to a further aspect of the present invention, the first annular chamfer has a face angle of greater than 8xc2x0.
According to another aspect of the present invention, the bonding tool is formed from a material containing at least 80% ZrO2 by weight.
According to yet another aspect of the present invention, the bonding tool is formed from a material selected from one of group consisting of i) ZrO2+Y2O3 and ii) Al2O3+ZrO2+Y2O3.